The present invention relates to a method of manufacturing a multiply-bent tubular element, particularly such which has, in its final shape, two coaxial end portions, a central portion which is axially parallel with the end portions, and transitory portions between the end portions and the central portion which have substantially S-shaped configurations. Also, the present invention relates to an apparatus capable of performing the above method. Tubular elements of the above-mentioned type find widespread use in the construction of various apparatus, and more particularly also in the construction of motor vehicles as constituents of the axles of such motor vehicles. Especially when the tubular elements are to be used in the last-mentioned application, they have to satisfy strigent quality requirements with respect to the accurate coaxiality of the end portions, on the one hand, and to the substantial conformity of the cross-sctions of the S-shaped transitory portions to their shapes prior to the deformation of the tubular element, on the other hand. While it is true that successful attempts have already been made to satisfy the above-discussed requirements, such has been achieved only by incurring substantial expenses in terms of time, material, machinery and personnel.
So, for instance, a prevalent practice has heretofore been to manufacture various curved portions of the tubular elements after one another. Under these circumstances, however, there are to be expected errors in angles, abutments, length and twists, caused by the performance of the operating steps, the type of the machinery used for the performance of the operating steps and the degree of skill and education possessed by the operating personnel controlling the progression of the bending operation, having regard for the rebounding effect taking place subsequently to the performance of the bending operations. However, these errors also occur when the bends of the tubular elements are manufactured in a single operation by means of dies which are equipped with appropriately curved surfaces. In order to be able to eliminate the above-discussed errors, resort must be had to correspondingly constructed arrangements to be operated by especially trained personnel familiar with the operation of such arrangement, in addition to the bending operations and equipment.
Furthermore, it has been heretofore necessary to calibrate the bent tubular elements after the elimination of the above-mentioned errors at least with respect to the S-shaped transitory portions in order to remove the ovality which develops at such transitory portions due to the bending of the tubular element thereat, so as to restore the original cross-sectional configuration of such transitory portions to the greatest extent possible. Even here, special machinery and operating personnel familiar with the operation of such special machinery are needed. When using this machinery, the operating personnel must be very careful when introducing the curved tubular elements from which the above-mentioned errors have already been removed, so as to properly position the tubular element therein. Were it otherwise, that is, were the curved tubular elements improperly introduced into the calibrating machinery, then errors of the above-mentioned type, but especially errors in angle, would again be produced in the improperly positioned curved tubular element. Under these circumstances, such errors, especially the angular errors, would have to be subsequently and additionally corrected at a great expense.
As already mentioned above, the employment of the known methods of manufacturing elongated curved tubular elements of the above-mentioned type by means of pipe-bending machines or by means of deformation by appropriately configurated dies, results in a situation where, for instance, an originally circular cross section of the tubular element is changed to an oval cross section during the bending of the tubular element at least at the S-shaped transitory portions of the tubular element, the ovality being more or less pronounced depending on the circumstances. In some instances, it is necessary, in order to keep the degree of ovality within the limits which are still suited for restoring the original circular cross section by calibration, to make the tubular wall of the tubular element relatively thick. In other words, more material is used for the tubular element than dictated by the consideration of, for instance, stability and strength when the tubular element is to be used as a load-carrying element, such as a motor vehicle axle, or by the resistance to flow when the tubular element is utilized in a fluid-flow circuit of a hydraulic or pneumatic apparatus.